Why does chaining not work here, and I must set an intermediary variable?

Question

I am relatively new to C#. I have the following excerpt of a script:

public class Flipper : MonoBehavior {
  HingeJoint hinge;
  void start () {
    hinge = GetComponent<HingeJoint>();
    hinge.limits.min = 0;
  }
}

The line hinge.limits.min = 0; does not compile; I need to instead do it in two steps, with intermediary variable assignment like so:

JointLimits limits = hinge.limits;
limits.min = 0

why is this the case?

`HingeJoint` doesn't implement the `min` property. `JointLimits` does. — Syntax Error, Sep 17 '17 at 22:50
You are implicit casting the object from `HingeJoint` to `JointLimits`. Also see: https://stackoverflow.com/questions/1339482/typecasting-in-c-sharp — Christian Gollhardt, Sep 17 '17 at 22:54

Programmer · Accepted Answer · 2017-09-18T04:15:28.943

There are two reasons for this:

1.JointLimits is a struct.

2.JointLimits is declared as auto-property variable.

HingeJoint.limits is a type of JointLimits which is a struct and declared as auto-property ({ get; set; }) so you can't modify a variable(min) that is inside it directly. You have to make a copy of the struct, modify the variable inside it then assign the struct back to HingeJoint. The use of struct and the auto property to declare the limits variable is why you can't do that.

public JointLimits limits { get; set; }

The-same thing applies to transform.position. You can't modify its x, y, z variables directly because position is a type of Vector3 which is a struct and it is also declared as auto-property:

public Vector3 position { get; set; }

You have to make a copy of it first, modify it then assign it back to transform.position.

Another unrelated problem in your code is the Start function. It is Start not start.

HingeJoint hinge;

void Start()
{
    hinge = GetComponent<HingeJoint>();

    //Make Limit copy
    JointLimits limits = hinge.limits;

    //Modify limit variable
    limits.min = 0;

    //Assign back to HingeJoint
    hinge.limits = limits;
}

Why does chaining not work here, and I must set an intermediary variable?

1 Answers1